Pyrolysis processes, such as steam cracking, are utilized for converting saturated hydrocarbons to higher-value products such as light olefins, e.g., ethylene and propylene. Besides these useful products, hydrocarbon pyrolysis can also produce a significant amount of relatively low-value heavy products, such as pyrolysis tar. When the pyrolysis is steam cracking, the pyrolysis tar is identified as steam-cracker tar (“SCT”). Hydroprocessing pyrolysis tar in the presence of a hydrogen-containing treat gas and at least one hydroprocessing catalyst produces an upgraded pyrolysis tar having improved blending characteristics with other heavy hydrocarbons such as fuel oil.
SCT generally contains relatively high molecular weight molecules, conventionally called Tar Heavies (“TH”). Catalytic hydroprocessing of undiluted SCT leads to significant catalyst deactivation. For example, a significant decrease in hydroprocessing efficiency is observed when hydroprocessing SCT at a temperature in the range of from 250° C. to 380° C., at a pressure in the range of 5400 kPa to 20,500 kPa, using (i) a treat gas containing molecular hydrogen and (ii) at least one catalyst containing one or more of Co, Ni, or Mo. The loss of efficiency has been attributed to the presence of TH in the SCT, which leads to the formation of undesirable deposits (e.g., coke deposits) on the hydroprocessing catalyst and the reactor internals. As the amount of these deposits increases, the yield of the desired upgraded pyrolysis tar (upgraded SCT) decreases and the yield of undesirable byproducts increases. The hydroprocessing reactor pressure drop also increases, often to a point where the reactor is inoperable.
It is conventional to lessen deposit formation by hydroprocessing the SCT in the presence of a utility fluid, e.g., a solvent having significant aromatics content. The upgraded SCT product generally has a decreased viscosity, decreased atmospheric boiling point range, and increased hydrogen content over that of the SCT feed, resulting in improved compatibility with fuel oil blend-stocks. Additionally, hydroprocessing the SCT in the presence of utility fluid produces fewer undesirable byproducts and the rate of increase in reactor pressure drop is lessened. Conventional processes for SCT hydroprocessing, disclosed in U.S. Pat. Nos. 2,382,260 and 5,158,668 and in International Patent Application Publication No. WO 2013/033590 involves recycling a portion of the hydroprocessed tar for use as the utility fluid.
The feed to the hydroprocessing reactor can be a mixture of SCT and utility fluid. It is conventional to recycle a portion of the liquid phase components of the hydroprocessor effluent as utility fluid. When doing so, it has been found to be sometimes necessary to add a supplemental utility fluid (e.g., Steam Cracked Naphtha (“SCN”)) to the feed to prevent deposits in the hydroprocessing reactor and/or SCT pre-heating equipment. This can be the case when the quality of the SCT in the feed changes sufficiently to result in an increase in the viscosity and/or final boiling point of the liquid phase components of the hydroprocessed effluent.
Since the supplemental utility fluid is itself a valuable product of the steam cracking process, there is a need for a SCT hydroprocessing process having a decreased need for supplemental utility fluid. It is particularly desired for such processes to produce an upgraded SCT having the desired properties at high yield over a broad SCT compositional range and/or a range of hydroprocessing temperature and pressure.